The document, xe2x80x9cAN Algorithm for Flow Rate Control of XTPxe2x80x9d by Jae K Song et al. (Proceeding of the International Conference on Communications (ICC), Geneva, May 23-26, 1993, Vol. 1 Engineers, May 23, 1993, pages 187-191, Institute of Elewctrical [sic] and Electornics [sic] Eingneers [sic], ISBN 0-7803-0950-2) discloses various methods for controlling the message flow.
Many communication protocols require a control circulation between transmitters and receivers such that the receiver is capable of controlling the transmission rate of the transmitter. This is often accomplished by explicit or implicit definition of a transmission window, whereby the transmitter is only allowed to have a specific plurality of non-acknowledged messages pending.
Specifically when the receiver can dynamically define the size of the window, this is thereby also called assigning credit by the receiver, i.e. the credit defines the upper limit of the transmission window. The receiver uses the assigning of credit to control the transmission stream according to the resources available to it, including the available reception buffers.
At least one protocol (Q.2110) that has already been defined uses the same messages for assigning credit that are also used for positive or, respectively, negative acknowledge. When, further, point in time and frequency of the acknowledgments are essentially controlled by the transmitter in such a method (as, for example, in Q.2110 where an acknowledgment only ensues on the basis of a prior request by the transmitter (exception: upon recognition of message losses, a negative acknowledgment is spontaneously sent by the receiver in Q.2110, i.e. without a request)), then the point in time and frequency of the modifications of the transmission window are also defined by the transmitter. This, however, runs essentially counter to the assumption that the control Substitute Page of the transmission window ensues by the receiver dependent on the resources available to it.
The mechanism for flow control explained above for the protocol Q.2110 is also known from the document, xe2x80x9cATM Volume IIxe2x80x9d Signaling in Broadband Networks, l/exe2x80x2 by U. D. Black xe2x80x9cAugust 1997, Prentice Hall PTR, United states)xe2x80x9d.
Due to said control by the transmitter, situations arise wherein the receiver can communicate a modification of the transmission window to the transmitter only after arrival of a corresponding (credit) request and not immediately after the occurrence of the modification. The disadvantages that arise are two-fold. First, fundamentally avoidable blockings of the message flow can occur due to delayed assigning of credit. On the other hand, an avoidable load on the receiver can occur due to the delayed credit reduction, as a result whereof, for example, message loss on other links and/or other connections that are controlled by the same reception means (receiving station) can occur.
The invention is based on the object of avoiding said disadvantages.
This object is achieved by a method according to claim 1.
The invention is described in greater detail below.
Inventively, the transmitter is informed of the currently desired transmission window even without a presence of message losses and without an explicit request by the transmitter (as, for example, with a POLL-PDU in Q.2110). This measure is applied in order to counter, on the one hand, blockings of the message flow and, on the other hand, overloads of the receiving station.
Specifically, this measure can be achieved, for example in Q.2110, in that, given necessary changes of the reception window, the receiver informs the transmitter thereof by a spontaneous transmission of what is referred to as a STAT-PDU. Another reason for an advantageous, spontaneous transmission of a STAT-PDU can, for example, also be comprised in spontaneously acknowledging (confirming) messages for the transmitter without having to wait for a request.
A particular advantage of the invention is to be seen therein that, even though it is not provided in Q.2110, a STAT-PDU can also be spontaneously transmitted by the receiver without a protocol infringement of Q.2110 thereby occurring at the transmitter. By applying the invention given Q.2110, thus, the above-described disadvantages are avoided without having to violate the protocol of Q.2110.
The application of the invention given Q.2110 is explained in brief below.
When the case of a desired change of the transmission window occurs, the receiver spontaneously transmits a STAT-PDU with the modified transmission window. Cases of a desired change are present, for example, when the current capacity of the reception buffer and/or the plurality of payload messages that a transmitter is still allowed to transmit unacknowledged for a connection falls below a specific value.
Further, a spontaneous STAT-PDU is sent when a gap in the reception buffer is closed by reception of a SD-PDU. As a result thereof, a possible blocking of the message flow is likewise opposed. Reception gaps are thereby not communicated. As usual, N(R) is occupied with the current value con VR(R). The POLL sequence number N(PS) is set to the value of the most recently received POLL-PDU (or, respectively, to 0 if a POLL_PDU has not yet been received). For easier administration, an additional receiver status variable VR(PS) can be introduced, this being initialized with 0 and being stored in the N(PS) of a received POLL-PDU.